1. Field of the Invention
The present invention relates to a semiconductor device typified by a thin film transistor and to a fabrication method thereof. Specifically, the present invention relates to a semiconductor device using a crystal silicon thin film formed on a glass substrate or a quartz substrate and to a fabrication method thereof.
2. Description of Related Art
Hitherto, there has been known a thin film transistor using a silicon film, i.e. a technology for forming the thin film transistor by using the silicon film formed on a glass substrate or quartz substrate.
The glass substrate or quartz substrate is used because the thin film transistor is used for an active matrix type liquid crystal display. While a thin film transistor has been formed by using an amorphous silicon film in the past, it is being tried to fabricate the thin film transistor by utilizing a silicon film having a crystallinity (referred to as xe2x80x9ccrystal silicon filmxe2x80x9d hereinbelow) in order to enhance its performance.
The thin film transistor using the crystal silicon film allows to operate at a high speed by more than two digits as compared to one using the amorphous silicon film. Therefore, while peripheral driving circuits of an active matrix liquid crystal display have been composed of external IC circuits, they may be built on the glass substrate or quartz substrate similarly to the active matrix circuit.
Such structure is very advantageous in miniaturizing the whole apparatus and in simplifying the fabrication process, thus leading to the reduction of the fabrication cost. In general, a crystal silicon film has been obtained by forming an amorphous silicon film by means of plasma CVD or reduced pressure thermal CVD and then by crystallizing it by implementing a heat treatment or by irradiating laser light.
However, it has been the fact that it is difficult to obtain a required crystalinity across the wide area through the heat treatment because it may cause nonuniformity in the crystallization.
Further, although it is possible to obtain the high crystalinity partly by irradiating laser light, it is difficult to obtain a good annealing effect across the wide area. In particular, the irradiation of the laser light is apt to become unstable under the condition for obtaining the good crystalinity.
Meanwhile, a technology described in Japanese Patent Laid-Open No. Hei. 6-232059 has been known. This is a technology which allows to obtain a crystal silicon film through a heat treatment at a lower temperature than that of the prior art by introducing a metal element (e.g. nickel) which promotes the crystallization of silicon to the amorphous silicon film.
According to studies conducted by the inventors et. al. it has been found that the crystal silicon film obtained by this method has crystalinity which is good for practical use across the wide area.
However, it has been also found that because the metal element is contained within the film and the amount thereof to be introduced has to be controlled very carefully, there is a problem in its reproducibility and stability (electrical stability of a device obtained).
Further, there is a problem that an elapsed change of the characteristics of a semiconductor device to be obtained is large or an OFF value, in case of a thin film transistor, is large, for example due to the influence of the remaining metal element.
That is, although the metal element which promotes the crystallization of silicon plays the useful role in obtaining the crystal silicon film, its existence becomes a minus factor which causes various problems after obtaining the crystal silicon film once.
It is an object of the invention disclosed in the present specification to provide a technology for reducing concentration of metal element within a crystal silicon film obtained by utilizing the metal element which promotes crystallization of silicon.
At first, how the present invention disclosed in the present specification has been devised will be explained in detail.
FIGS. 6 and 7 are graphs of measured values of gate current of a planar type thin film transistor which the inventors et. al. have made in trial. FIG. 6 is different from FIG. 7 in that whether thermal oxidation is used or plasma CVD is used in forming a gate insulating film. That is, FIG. 6 shows measured values obtained when the gate insulating film has been formed by the thermal oxidation and FIG. 7 shows measured values obtained when the gate insulating film has been formed by the plasma CVD.
The horizontal axis in FIGS. 6 and 7 represents the gate current and the vertical axis represents a number of measured samples.
A quartz substrate is used as the substrate here. Further, an active layer is formed by holding nickel element in contact with the surface of the amorphous silicon film and by crystallizing by a heat treatment of four hours at 640xc2x0 C. The thermal oxide film is formed within an oxygen atmosphere at 950xc2x0 C.
It can be seen from FIG. 6 that the values of gate current vary largely depending on the samples. It shows that there is dispersion in the quality of the gate insulating film.
Meanwhile, as shown in FIG. 7, there is less dispersion of the gate currents and its value is extremely small in the thin film transistor in which the gate insulating film has been formed by the plasma CVD.
The reason why the difference of the measured values shown in FIGS. 6 and 7 appears may be explained as follows. That is, nickel element is drawn from the active layer to the thermal oxide film when the thermal oxide film is formed in the samples in which the gate insulating film is formed by the thermal oxide film. As a result, nickel element which hinders the insulation comes to exist within the thermal oxide film. The existence of the nickel element increases a value of current leaked within the gate insulating film and varies that value.
This fact is supported by SIMS (secondary ion mass spectrometry) and by measuring the concentration of nickel element within the gate insulating film of the samples whose measured values have been obtained in FIGS. 6 and 7.
That is, it was confirmed that while nickel element of more than the level of 1017 cmxe2x88x923 is measured within the gate insulating film formed by the thermal oxidation, the concentration of nickel element within the gate insulating film formed by the plasma CVD is less than the level of 1016 cmxe2x88x923.
The present invention disclosed in the present specification is based on the findings described above. That is, the thermal oxide film is formed on the surface of the crystal silicon film obtained by utilizing the metal element which promotes the crystallization of silicon to getter the metal element within the thermal oxide film and to reduce the concentration of the metal element within the crystal silicon film as a result.
One of the invention disclosed in the present specification is characterized in that it comprises steps of intentionally introducing metal element which promotes crystallization of silicon to an amorphous silicon film and crystallizing the amorphous silicon film by a first heat treatment to obtain a crystal silicon film; eliminating or reducing the metal element existing within the crystal silicon film by implementing a second heat treatment within an oxidizing atmosphere; eliminating a thermal oxide film formed in the previous step; and forming a thermal oxide film on the surface of the region from which the thermal oxide film has been eliminated by implementing another thermal oxidation.
Another constitution of the invention is characterized in that it comprises steps of intentionally introducing metal element which promotes crystallization of silicon to an amorphous silicon film and crystallizing the amorphous silicon film by a first heat treatment to obtain a crystal silicon film; eliminating or reducing the metal element existing within the crystal silicon film by implementing a second heat treatment within an oxidizing atmosphere to form a thermal oxide film on the surface of the crystal silicon film and by causing the thermal oxide film to getter the metal element; eliminating the thermal oxide film formed in the previous step; and forming a thermal oxide film on the surface of the region from which the thermal oxide film has been eliminated by implementing another thermal oxidation.
Another constitution of the invention is characterized in that it comprises steps of intentionally introducing metal element which promotes crystallization of silicon to an amorphous silicon film and crystallizing the amorphous silicon film by a first heat treatment to obtain a crystal silicon film; eliminating or reducing the metal element existing within the crystal silicon film by implementing a second heat treatment within an oxidizing atmosphere; eliminating thermal oxide film formed in the step; forming an active layer of a thin film transistor by implementing patterning; and forming a thermal oxide film which composes at least a part of a gate insulating film on the surface of the active layer by means of thermal oxidation.
Another constitution of the invention is characterized in that it comprises steps of selectively introducing metal element which promotes crystallization of silicon to an amorphous silicon film; growing crystal by a first heat treatment in a direction parallel to the film from the region to which the metal element has been selectively introduced; forming a thermal oxide film on the surface of the region where the crystal has been grown by implementing a second heat treatment within an oxidizing atmosphere; eliminating the thermal oxide film; and forming an active layer of the semiconductor device by using the region from which the thermal oxide film has been eliminated.
Another constitution of the invention is characterized in that a semiconductor device has a crystal silicon film interposed between first and second oxide films; the crystal silicon film contains metal element which promotes crystallization of silicon; and
the metal element is distributed in high concentration near the interfaces with the first and/or second oxide film within the crystal silicon film.
An arrangement of another invention is characterized in that a semiconductor device comprises an underlying layer made of an oxide film; a crystal silicon film formed on the underlying layer; and a thermal oxide film formed on the crystal silicon film; wherein the crystal silicon film contains metal element which promotes crystallization of silicon; the metal element which promotes the crystallization of silicon is distributed in high concentration near the interface with the underlying layer and/or the thermal oxide film; and the thermal oxide film composes at least a part of a gate insulating film of a thin film transistor.
In the invention disclosed in the present specification, one or a plurality of metal elements selected from Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu and Au may be used as the metal element which promotes the crystallization of silicon.
The concentration of impurity in the present specification is defined as the minimum value of measured values measured by the SIMS (secondary ion mass spectrometry).